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Mozilla yesterday took another step toward delivering a "Metro" version of Firefox to Windows 8 users.

Late Tuesday, Asa Dotzler, the Firefox desktop product manager, announced that a preliminary Metro browser had reached "mozilla-central," the source code repository that feeds into what Mozilla calls the "Nightly" build channel.

Nightly builds are designed for testing, and as the name implies, are automatically updated each night to that day's edition. Firefox's Nightly builds are Mozilla's roughest-edged editions. Every six weeks, the current Nightly morphs into an Aurora build -- analogous to an alpha -- that kicks off a three-month development cycle which ends when the next iteration is officially released.

"If you are on the Firefox Nightly channel and you have a Windows 8 device, your Wednesday Firefox update should deliver a Metro Firefox tile to the far right end of your Windows Start screen," Dotzler wrote.

Like many, Dotzler used "Metro" to refer to the kind of apps that run in the new tile-based user interface (UI) of Windows 8, even though Microsoft abandoned the label last summer.

"There's plenty of work still to do, but it's stable enough that we're ready for more and more regular testing," added Dotzler.

Mozilla debuted a Metro-ized Firefox preview last October, weeks before Microsoft launched Windows 8. To run the preview, people had to explicitly download it from a different, and experimental, development repository. Since then, the open-source developer has been quiet about its work on Windows 8 and Metro.

Today's addition of Metro Firefox to mozilla-central means that all Nightly users running Windows 8 will be updated daily to the latest version at approximately 9:30 p.m. PT. The move will expose a larger group of testers to the Metro version.

Mozilla still has not set a timetable for a finished, polished Windows 8 browser that includes both a desktop version and one for the Metro UI. In a month-old message posted to the Metro Firefox mailing list, Dotzler said that the goals for the first quarter -- which ends March 31 -- were to integrate the version with mozilla-central, and "deliver a preview/milestone that has a feature-complete Awesome screen experience."

Browsers are a special case for Windows 8. Their Metro-ized versions can run outside the normal security sandbox, and have access to most Windows APIs (application programming interface) on the classic desktop, as well as the new WinRT API, the backbone of Windows 8's Metro app development.

The category also gets other passes from Microsoft: A desktop browser with a Metro version circumvents the Windows Store, and when installed on the Windows 8 classic desktop, simultaneously installs the Metro version.

The biggest caveat for a Windows 8 browser is that only the default browser can run in the Metro UI. During setup, Windows 8 assigns Internet Explorer 10 (IE10) as the default browser. However, users can change the default.

The Nightly build of Firefox can be downloaded from Mozilla's website. To use Metro Firefox, of course, users must install the Nightly on a PC running Windows 8 or on a tablet, such as Microsoft's Surface Pro, powered by the OS.